The idea of a “progressive alliance” – which has been pushed by senior figures in Labour, as well as the Greens and Liberal Democrats – would see some left or centre-left parties not stand candidates in seats where splitting the vote could allow the Conservatives to succeed.

In a letter to the Labour and Lib Dem leaders, Ms Lucas and Mr Bartley said there was “a role for some form of cooperation in a handful of seats to create the best possible chance of beating the Tories and, crucially, of thereby delivering a fairer voting system”.

Ms Lucas, the MP for Brighton Pavilion, added: “Britain is at a crossroads – and this election will dictate the very future of our country.

“The Green party will be standing on a unique policy platform – opposing the Tories’ Brexit and putting forward big ideas for a fairer economy and the protection of our environment.

“Our call for a meeting between party leaders isn’t about the Greens standing aside – it’s about giving people in this country the best possible chance of defeating the Conservatives and bringing in a truly democratic voting system.

“For the sake of our NHS, our welfare state and our environment we need progressive party leaders to ditch partisan politics just for a moment and think about how we can best stop the Tories from wrecking our country for generations to come.”

Labour MPs Jonathan Reynolds, Lisa Nandy and Clive Lewis were publicly slapped down by the leadership when they called for the party not to put up a candidate in Richmond Park.